This invention relates generally to auxiliary power steering systems for automotive vehicles.
Auxiliary power steering systems with which the present invention is associated, is of a type having a servomotor pump, a pressure storage reservoir, a steering control valve blocking flow in its neutral position and operable by a hand steering wheel, at least one piston servomotor provided with two pressure operating chambers and a fluid return tank. In such systems, the servo pump and the pressure storage reservoir usually constitute the fluid pressure source from which operating pressures of 140 to 180 BAR are derived. By use of such an auxiliary power steering system, all motor vehicle operational systems including braking, steering, level regulations, etc. may be served by the same pressurized fluid supply. In such case, the pressure storage reservoir is prestressed to maintain a high operating pressure for the systems. Furthermore, a servo pump of small capacity may be selected inasmuch as the reservoir acts as a pressurized fluid collector. In order to serve several fluid operating systems by means of such a common fluid pressure source, it is necessary for the steering control valve of the auxiliary power system to be locked in its neutral, flow blocking position.
A serious problem arises in the foregoing arrangement from the loss of fluid by leakage in view of the high continuous operating pressure prevailing at the valve control edges of the steering control valve. While overlapping of valve surfaces at the control edge portions is desired to obtain a better steering sense from the steering control valve, a high leakage loss occurs in such case. Furthermore, the deposit of solid particles and dirt within the steering control valve impairs tight fitting relationships and thereby increases losses through leakage.
Another disadvantage associated with auxiliary power steering systems of the foregoing type arises from the establishment of variable pressure differentials between the fluid pressure at the inlet of the steering control valve and the pressure prevailing in the operating chambers of the piston servomotor. As a result of the variations in pressure differentials, the steering behavior of the vehicle suffers.
It is therefore an important object of the present invention to provide an auxiliary power steering system wherein fluid flow may be blocked in the neutral position of the steering control valve with minimal leakage and a constant pressure differential maintained between pressures at the inlet and outlet ports of the control valve.